What are some traits and assumptions of "formalist" literary criticism?

“Formalist” criticism, as the term implies, is chiefly concerned with the “form” of a literary work rather than with its “content.” In other words, formalists tend to be more concerned with how a work is written than with what it “says.” They are the first to insist, however, that “form” and “content” cannot really be separated. To take a trivial example, the phrase “let’s eat grandma!” is significantly different in meaning from the phrase “let’s eat, grandma!” merely because of the addition of a single comma. This example aptly illustrates that main formalist claim that any detail of the phrasing of a text, no matter how apparently small, can significantly affect the meaning of the text.

A more important example involves the famous line from William Shakespeare’s Hamlet in which the title character refers to his “too too _____ flesh.” Which word should fill the blank? As Amanda Mabillard notes,

Many scholars ask whether Shakespeare intended "solid" to be actually "sallied", a form of the word "sullied." The second quarto of Hamlet contains "sallied", but the First Folio prints it as "solid." Modern editors have been quite divided on the issue. Editors of The Arden Shakespeare choose to use "sullied", while editors of The New Cambridge Shakespeare have decided upon "solid." [see link below]

To a formalist, matters such as this – matters involving a single word or even a single letter or single sound – are highly important. Formalists often think of texts as “complex unities,” in which both complexity and unity are important. A text is like a puzzle in which every part is important in making up the whole. The chief task of any formalist critic is to show how the parts are related to the whole, a vice versa.

It is the emphasis on “wholeness” and “unity” that has made formalism the object of attack from so many other, more recent theories. Many of these other theories argue that works of literature rarely display true unity, and therefore that any unity “found” in them is actually imposed upon them. Critics of formalism argue that formalist readings of texts are boringly predictable: some alleged “unity” will always be the result of the process. Critics of formalism also often argue that formalism operates in a vacuum – focusing on “autonomous texts” while ignoring relevant economic, historical, political, sexual, cultural (etc.) contexts.

Defenders on formalism (which seems to be in a “comeback” phase after having been attacked for half a century) argue, in response, (1) that formalists never neglected contexts as much as critics have claimed; (2) that in any case formalism can work just fine (if not better) in conjunction with concerns about contexts; (3) that formalist analyses are in fact often far less predictable than readings offered by other schools, precisely because formalists care about the tiniest details; (4) that formalism pays attention to the traits that make “literature” literature (that is, the traits that make a piece of writing call attention to itself as a piece of writing); and (5) that formalism is in some ways simply an extension of rhetorical criticism, which has an ancient and honorable pedigree.